J. Synchrotron Rad. (2001). 8, 809±811 # 2001 International Union of Crystallography  Printed in Great Britain ± all rights reserved 809 amorphous, glass and liquids Local atomic and electronic structure of Al 90 Fe x Ce 10-x alloys: XAFS analysis A. N. Mansour, a* G. Cibin, b A. Marcelli, b T. Sevastyanova, c G. Yalovega, c and A. V. Soldatov c a Naval Surface Warfare Center, Carderock Division, West Bethesda, MD 20817-5700, USA, b LNF, INFN Frscati, Italy, c Faculty of Physics, Rostov State University, 5 Sorge, Rostov-na-Donu, 344090, Russia. Email: MansourAN@nswccd.navy.mil X-ray absorption fine structure (XAFS) above the Fe K-edge and the Ce L 3 -edge in amorphous Al 90 Fe x Ce 10-x (x = 3, 5, and 7) alloys have been measured and analyzed. Quantitative analyses of the Fe K-edge and Ce L 3 -edge EXAFS spectra are limited to local structure parameters of the first coordination sphere. Using a theoretical multiple scattering (MS) approach, we show that the Fe and Ce XANES are sensitive to the structure of coordination spheres, which extend up to nearly 4.5 Å. Comparison of experimental XANES spectra with theoretical MS results allows one to determine the local structure around the iron and cerium sites up to at least the third shell of atoms. Keywords: Aluminium-based alloys, Al-Fe-Ce, amorphous metals, X-ray absorption spectroscopy, XANES, EXAFS. 1. Introduction Liquid-quenched amorphous Al 90 Fe x Ce 10-x (x = 3, 5, and 7) alloys combine the properties of a metal with the short-range order of a glass (Hseih et al., 1990 & 1991). Their structural characteristics led to a number of remarkable mechanical (Shiflet et al., 1988), magnetic (Wagner et al., 1989), and corrosion (Mansour & Melendres, 1995; Mansour et al., 1996) properties. These alloys have high strength, high ductility, low density, and high resistance to corrosion. Determination of the atomic and electronic structure of these alloys is essential in order to understand the origin of these properties. Therefore, the structure of these alloys was investigated by pulsed neutron and x-ray scattering methods (Hseih et al., 1990, 1991). The structure of vapor-quenched amorphous Al 100-2x Co x Ce x (x = 8, 9, and 10) and Al 80 Fe 10 Ce 10 was investigated by x-ray absorption fine structure (XAFS) analysis (Mansour et al., 1994). In this investigation, the Fe K-edge and the Ce L 3 -edge XAFS of amorphous Al 90 Fe x Ce 10-x (x = 3, 5, and 7) alloys have been measured and analyzed. The goal is to determine the best model for the local structure of the Al 90 Fe x Ce 10-x system and to study the peculiarities of the electronic structure of these alloys using XAFS analysis. Fourier transforms of the Fe K-edge and Ce L 3 - edge EXAFS spectra (not shown here) display prominent contributions from the first coordination spheres of Fe and Ce, respectively, with no significant contributions from higher coordination spheres due to the amorphous nature of these alloys. Hence, quantitative analyses of EXAFS spectra are limited to the first coordination spheres of Fe and Ce (Table 1). The local structure parameters for Fe are independent of the composition range investigated here. Fe is coordinated with approximately 6.6 Al atoms at a distance of 2.46 Å. The disorder for the Fe-Al sphere is within the range expected for crystalline materials such as those for metallic Cu and Fe (Sevillano et al., 1979). The Ce- Al coordination sphere, on the other hand, consists approximately of 14 Al atoms at a distance which increased from 3.13 Å to 3.18Å in going from x = 7 to 3. The Ce-Al coordination is characterized by a high degree of structural disorder suggesting a distribution of Ce-Al distances in close proximity of each other. For comparison purposes, we have also included the structure parameters for crystalline Al 90 Fe 3 Ce 7 . Clearly, the Fe-Al distance increased from 2.46 to 2.56 Å in going from a-Al 90 Fe 3 Ce 7 to c- Al 90 Fe 3 Ce 7 without a significant change in the Fe-Al coordination and disorder. The Ce-Al distance increased from 3.18 to 3.25 Å in going from a-Al 90 Fe 3 Ce 7 to c-Al 90 Fe 3 Ce 7 without a significant change in coordination. However, the Ce-Al disorder decreased from 0.0206 to 0.0081 Å 2 in going from a-Al 90 Fe 3 Ce 7 to c- Al 90 Fe 3 Ce 7 indicating a higher degree of structural disorder for the Ce-Al relative to that of the Fe-Al environment in the amorphous state. Table 1 Structure parameters for Fe and Ce as determined from analysis of the Fe K-edge and the Ce L3-edge EXAFS spectra for amorphous (a) and crystalline (c) alloys. N, R, and σ 2 are the coordination number, distance, and mean square relative displacement, respectively. N and σ 2 are accurate to ± 10% and R to ± 0.02 Å. The c-Al90Fe3Ce7 alloy was obtained by heating the a-Al90Fe3Ce7 alloy. Sample X-Y Pair N R (Å) σ 2 (10 -3 Å 2 ) a-Al 90 Fe 7 Ce 3 Fe-AL Ce-Al 6.6 14.0 2.46 3.13 7.7 18.5 a-Al 90 Fe 5 Ce 5 Fe-AL Ce-Al 6.6 13.4 2.46 3.15 7.5 18.7 a-Al 90 Fe 3 Ce 7 Fe-AL Ce-Al 6.7 13.9 2.46 3.18 7.7 20.6 c-Al 90 Fe 3 Ce 7 Fe-AL Ce-Al 7.7 12.3 2.56 3.25 8.4 8.1 Hence, the goal is to explore the XANES region and determine if extra structural information with regard to higher coordination spheres can be obtained on the basis of multiple scattering (MS) analysis. The MS approach used in this investigation has been successfully applied in order to interpret a large number of XANES for various materials (Durham, 1989; Fujikawa et al., 1983; Vvedensky et al., 1986; Della Longa et al., 1995; Vedrinskii et al., 1998; Rehr & Albers, 2000). The analysis has been found rather powerful for the case of ordered alloys like Ni 3 Al and NiAl 3 (Mansour et al., 1997) but no such analysis has been applied to the study of amorphous Al 90 Fe x Ce 10-x alloys. We show that analysis of XANES data enabled us to choose the best structural model including symmetry for the alloys investigated. 2. Experiment and Method of Calculation Amorphous samples in the form of 25 μm thick ribbons with nominal compositions Al 90 Fe x Ce 10-x (x = 3, 5, and 7) were prepared by rapid solidification from the liquid phase (He et al., 1988). The room temperature Fe K-edge and Ce L 3 -edge XAFS spectra were measured in transmission mode on beamline X-11A at the National Synchrotron Light Source. Details of experimental setup were published elsewhere (Mansour et al. 1996). The energy resolution in the measured interval is about 1.0 eV. The Fe K-edge and Ce L 3 -edge XANES for the amorphous alloys are essentially the same in the composition range investigated here. The algorithm of the scattering wave method was described earlier (Della Longa et al., 1995). The local structure around the iron and cerium sites of Al 90 Fe x Ce 10-x alloy was treated using structural models based on crystalline FeAl 6 and CeAl 4 (Villars & Calvert, 1985) and FeAl 3 (Black, 1955). The structure parameters for the Fe clusters used in the calculations are reported in Table 2 (crystalline FeAl 3 ) and in Table 3 (crystalline FeAl 6 ). Fe in FeAl 3 is present in five non-equivalent sites (see Table 2). In this case, the theoretical spectrum represents the weighted average of contributions from all non-equivalent sites. The theoretical spectrum for FeAl 6 was also calculated using actual atomic distances reduced by 3.4%. The reduced atomic distances provide